While features of the invention are subject to a wide range of applications, the invention is especially suited for use in TASI (Time Assignment Speech Interpolation) systems and will be particularly described in that connection.
TASI systems have been used to improve the utilization efficiency of voice communications systems by reducing the time that telephone lines are temporarily idle. For example, in a conventional two-way telephone circuit, over 50% of the line's capacity is wasted, accommodating the listener's transmit channel.
The TASI system constantly monitors speech channels and quickly reassigns lines from idle channels to active channels increasing the overall efficiency of the system. TASI systems have been described in the literature; for example, K. Bullington and J. M. Fraser, "Engineering Aspects of TASI", BSTJ, Vol. XXXVIII, March 1959, and "Transmission Systems for Communications", Bell Telephone Laboratories, 1970, pages 682 to 684 including references.
In the conventional TASI system, separate line(s) are used for transmitting channel assignment information, although systems have been described wherein the initial channel assignment information is transmitted via the line assigned to transmit the channel.
It is noteworthy that the telephone communications network systems utilize a multiplicity of transmission systems; including, wire line, wideband cables, Satellite links, microwave radio links, etc., and that the time delay characteristics of these transmission systems substantially differ. Such significant differences in delay can cause degraded control performance of a TASI system when, say, long time delay lines are used in transmitting control information of short time delay lines. Also, failure of a line transmitting control information can cause a large number of circuits to improperly perform. Thus, a conventional TASI system may be subject to poor operation and simultaneous interruption of a number of conversations if it utilizes a common control line.
If TASI systems utilize the assigned line to transmit the initial channel assignment information, the problem of a difference in time delay between the line carrying the voice signal and the line carrying the control information is eliminated except for poor timing of the line disconnect system. Also, since conventional TASI systems transmit information regarding channel assignment status and idle line status over a separate line, these systems are subject to performance degradation when the control line becomes inoperative. Of course, spare lines can be provided, but at a loss in line utilization efficiency. Furthermore, if a line carrying a telephone signal fails in such systems, a conversation is interrupted until the call is reassigned.